The present application claims the benefit of 35 U.S.C. 119(a) of EP Ser. No. 98302374.8 filed Mar. 27, 1998, and entitled METHOD AND APPARATUS FOR DRILLING A PLURALITY OF OFFSHORE UNDERWATER WELLS.
The present invention relates to a method and apparatus for drilling a plurality of wells.
Two conventional methods exist for drilling an offshore underwater well. The first of these is to drill and set a conductor pipe between a surface platform and the sea bed followed by drilling a surface well using a platform wellhead. The Blow Out Preventer (BOP) is located on the surface wellhead. Subsequent casing strings are landed in the surface wellhead. The well is completed by suspending completion tubing from the wellhead and installing a platform tree. A second method is to drill and set a conductor pipe into the seabed using a floating drilling vessel with the wellhead located on the bed. A subsea drilling BOP has to be run on a drilling riser down to the seabed and is connected to the subsea wellhead. A subsea well is drilled with casing hangers landed in the subsea wellhead followed by the tubing completion the well is completed by placing a tree on the seabed wellhead. An alternative subsea option is to use a horizontal tree and then run the tubing.
As the industry moves further offshore and beyond the continental shelf, the water depths being considered are drastically increasing as reservoirs down the flank of the continental shelf and on the ocean floors are discovered. These water depths rule out the use of conventional platforms and their low cost drilling techniques. Floating or tension production platform systems can be used but their drilling footprint into the reservoir is limited, requiring peripheral seabed subsea production support wells. Subsea fields involve considerable complex subsea architecture and require extensive high cost rig intervention.
One way in which an attempt has been made to increase the footprint of a production platform is the provision of a slanted conductor. In such an arrangement, the conductor is supported at an angle by the platform so that it can be run in at an angle thereby increasing the lateral distance between the base of the platform and the location where the conductor meets the seabed. However, such an arrangement is awkward and costly as it requires a specially made structure to support the conductor at an angle. Further, the system will not work in deep water without some support for the conductor at various locations between the surface and the seabed which is not available from a floating platform.
Our co-pending application (Agent""s Ref: PAJ07074EP), corresponding to U.S. patent application Ser. No. 09/275,346 filed Mar. 24, 1999 entitled Method and Apparatus for Drilling an Offshore Underwater Well, filed on the same day as the present application discloses a method of drilling an offshore underwater well comprises the steps of installing a riser conduit so that it is substantially vertically supported at a production deck situated substantially at the sea surface and deviates progressively further from the vertical with increasing sea depth, fixing the riser conduit at the seabed in a nonvertical orientation, and drilling the well into the seabed at an angle to the vertical.
As the riser conduit is substantially vertically supported at the production deck, it is possible to use conventional platform drilling and production techniques which help keep the drilling costs to a minimum. Further, because the riser conduit is supported at the surface and at the seabed, and deviates progressively further from the vertical in between, intermediate support is not required but can be provided if necessary by buoyancy modules.
In some fields, the reservoir could be relatively close to the seabed. In such a case, there is insufficient depth for a conventional subsea well which starts vertically at the seabed to be deviated to a sufficient angle to access reservoir formations not already being drained by nearby vertical or deviated wells. Therefore only a limited reservoir acreage can be accessed. With this arrangement some of this deviation from the vertical is already provided before reaching the seabed, so that less deviation is required underground which allows higher angle or horizontal wells to be drilled far along the reservoir. This allows better access to reservoirs which are close to the seabed. However, the most important benefit of this arrangement arises when the water is sufficiently deep that the riser conduit can be deviated to be horizontal at the seabed. Once the riser conduit becomes horizontal, it is possible to extend it some considerable distance along the seabed before drilling into the seabed so that the drilling footprint of a platform can be greatly increased without drilling.
The present invention relates to an improvement of the method and apparatus of our co-pending application.
Although the system of the co-pending application represents a vast improvement on the prior art in terms of being able to increase the size of the footprint of a platform, it does require the riser conductor to be able to contain the full production pressure and over riser conductor per well.
According to the present invention a method of drilling and completing an underwater well comprises the steps of installing a drilling riser conduit which is vertical at the sea surface to the seabed with the lower end of the drilling riser conduit connected to a template having an inlet port to which the lower end of the drilling riser conduit is connected and a wellhead accessed through the inlet port, such that the drilling riser conduit is at an angle to the vertical at the seabed; fixing the template to the seabed; drilling into the seabed through the wellhead in the template at an angle to the vertical; landing and sealing the well casing and a completion string within the wellhead; and installing a valve tree in the template to direct the flow of production fluid to the surface along a line separate from the drilling riser conduit.
As the wellhead is now at the seabed and the production fluid flows to the surface through a line separate from the drilling riser conduit, it is no longer necessary to have a wellhead at the platform, nor is it necessary for the drilling riser conduit to be lined to take the full reservoir pressure.
There is also preferably provided within the template means for receiving a BOP for installation during well drilling and completing.
A method of landing and locating various components, such as the valve tree and/or the BOP is to lower the components on a skid into the template, and then extend connecting elements together to seal inlet and outlet ports of the components in place.
The well casing is preferably centred in the wellhead by radially projecting centring members.
A further drawback with the system of the co-pending application is that it requires one riser conduit per well. This can be a problem for a large reservoir as each riser conductor requires one well slot on the platform. The hanging loads caused by the casing strings and the heavy mud columns will require high deck support from a large tension leg platform when a large number of wells are being drilled and completed. In addition the drilling range with this concept is limited to the maximum drilling reach from a single point. A large field would now require several platform systems or revert back to using a subsea field system for distant wells.
One major benefit of the present invention arises when several wells can be drilled from a single template. In this case, the template is a junction template provided with a plurality of outlet ports each associated with its own wellhead and valve tree, and a port selector is provided for selectively connecting the inlet port with any one of the outlet ports, the method further comprising drilling into the seabed selectively through more than one outlet port using the port selector selectively to provide access to each outlet port.
This method allows a plurality of wells to be drilled from a single drilling riser conduit.
The step of drilling through the outlet port may either be done directly into the seabed, or may be indirectly done when the above junction template is a first stage junction template through one or more second stage junction templates, each having an inlet port, a plurality of outlet ports, and a port selector for selectively connecting the inlet port with any one of the outlet ports, at least one of the outlet ports of the first stage junction template being connected by a drilling conduit to the inlet port of a second stage junction template. It is possible for the second stage junction templates to be connected in a similar way to one or more third stage junction templates each having an inlet port, a plurality of outlet ports, and a port selector for selectively connecting the inlet port with any one of the outlet ports, such that a branched configuration comprising numerous wells can be constructed in order to cover a large area of a reservoir using only a single drilling riser conduit. Additional stages of junction templates can be added if necessary.
With the method of the present invention, it will often be the case that pipes have to be run down the drilling riser conduits and drilling conduits to the well templates on a running tool. The pipes will have to pass along significant lengths of horizontal drilling riser conduit. According to a further aspect of the present invention, there is provided a method of propelling a running tool and associated piping along a horizontal section of conduit, the running tool being provided with at least one piston element between the piping and a drilling installation, the outer diameter of the piston being substantially equal to the inner diameter of the conduit, so that the running tool slides through the drilling riser conduit and a piston seals with the drilling riser conduit; the method comprising the step of introducing hydraulic fluid into the drilling riser conduit behind of the piston member in order to push the piston member and hence the running tool along the conduit.
Preferably, several pistons are provided in series to distribute the load over all of the pistons and to ensure that they maintain a propulsive force on the running tool even if the seal of an individual piston loses its integrity.
Preferably a utility line extends from the drilling installation to meet the internal bore of the drilling conduit at a location beyond of the most advanced location of the piston closest to the running tool, and at least one valve is provided to control fluid flow through the utility line. This utility line can be used to accommodate fluid displaced by the pistons when the running tool is run in, and also can be used to provide hydraulic pressure on the downstream side of the or each piston so as to assist with a withdrawal of the running tool.
The template forms an independent aspect of the present invention which can be broadly defined as a template for a subsea wellhead assembly the template comprising a main body, means for fixing the main body to the seabed, an inlet port for receiving a drilling riser conduit at an angle to the vertical, a wellhead inclined at an angle to the vertical, and being accessible through the inlet port, and means for receiving other wellhead components such as a valve tree and BOP aligned such that they can be accessed through the inlet and allow access to the wellhead.
The orientation of the inlet port and wellhead is preferably such that, when the template is fixed to the seabed, the inlet port and wellhead are substantially horizontal.
The template is preferably provided with at least one bay for receiving various well components such as the valve tree and/or BOP, each component being mounted on a skid, and being extendable to locate and seal in the template.
In the case of the junction template, there is preferably further included a plurality of outlet ports, and a port selector for selectively communicating the inlet port with each of the plurality of outlet ports.
The template may be in two parts, one housing the wellhead and other wellhead components, the other housing the port selector. This helps reduce the size of individual components.
The orientation of the inlet and outlet ports and the means for anchoring the main body is preferably such that, when the junction template is fixed to the seabed, the ports open substantially horizontally.
The convenient method of fixing the junction template to the seabed has been found to be by using a gravity base or piles.
According to a further aspect of the present invention there is provided an apparatus for drilling a plurality of underwater wells, the apparatus comprising a drilling riser conduit extending from the sea surface to the seabed, such that the drilling riser conduit is at an angle to the vertical at the seabed, a junction template as defined above anchored to the seabed, wherein the drilling riser conduit is connected to the inlet port of the junction template, and wherein a plurality of drilling conduits extend across the seabed, and/or a plurality of conductor pipes extend into the seabed, from the outlet ports of the junction template.
When a drilling conduit extends from an outlet port, it is connected to the inlet port of a second stage junction template as defined above. The second stage junction template may also have both drilling conduits and conductor pipes extending from its outlets with one or more further stages of junction templates according to the second aspect of the present invention being connected in a similar way to each drilling conduit.
If the production fluid is to flow to the surface through the drilling riser conduit, it is necessary to provide a pressure containing casing type within the drilling riser conduit. However, the preferred alternative is to provide in the well template for each branch at which a well is drilled a subsea valve tree which is preferably a horizontal valve tree. Thus, the flow from each well can be brought to the surface externally of the drilling riser conduit in a conventional flow line.